1. Technical Field
The present invention relates to the field of signal converters used with electric motor drives and, more particularly, to one comprising an operational amplifier and an integrator, the output signal of which is a reference input signal for the operational amplifier.
2. Description Of The Related Art
Signal converters are used with electric motor drives to provide, for example, motor speed or armature current output signals to microprocessors. Signal converters for such arrangements are commercially available which are required for the potential-isolated further processing of measured motor speed values. For instance, it is known to further process a reference value and actual measured values of the motor speed. No data losses should occur in this further processing. In the data processing activity the measured signal level is converted into a frequency proportional to the level of the measured signal and is thus transmitted as a frequency and not as a voltage to the data processor. On the low-voltage side of the drive, this frequency is again converted into a voltage proportional to the frequency. It has been found that especially in the case of a high common mode, the linearity of these signal converters is not sufficient. Offset compensation is necessary especially if inexpensive fast precision amplifiers are employed for realizing the integrator.
From the German Offenlegungsschrift No. 34 24 052, a signal converter with a first integrator comprising an operational amplifier is known which is followed by a threshold level setting device. The first integrator is coupled to the measured signal input to the converter. The output of the threshold level setting device actuates a switching element. The output of the switching element is coupled to the summing point of the first integrator. In addition, a second integrator also comprising an operational amplifier is provided, the summing point of which is also tied to the measured signal input and to the output of the switching element. The output signal of the second integrator is the input reference signal of the first integrator. Thereby, the signal converter is given reasonable linearity because a correction signal is formed by the second integrator which makes the first integrator track until it is assured that the output signal corresponds to the input signal exactly.
However, it is still necessary to improve the linearity and resolution of such arrangements so that the digital sampling of analog output signals can also be improved and the further digital processing improved thereby.